1. Field of the Invention
This invention-relates to an X-ray detecting device, and more particularly to a liquid crystal display (LCD) X-ray detecting device that is capable of preventing a short between a lower electrode of a capacitor and a data line. Also, the present invention is directed to a method and apparatus for fabricating the X-ray detecting device.
2. Description of the Related Art
Diagnostic X-ray sensor imaging systems, which irradiate X-rays rather than visible light onto an object to photograph an image, are widely used in areas such as medical fields. An X-ray sensor requires a device for detecting the X-ray.
Recently, studies have been conducted where an active matrix liquid crystal display (LCD) is used in the X-ray detecting device. The active matrix LCD uses a thin film transistor (TFT) as a switching device.
Such an X-ray detecting device as described above includes a photo sensitive layer for detecting an X-ray and a thin film transistor substrate for switching and outputting the detected X-ray from the photo sensitive layer. The sensitive layer is formed from selenium, as described in Korean Patent Application No. 1999-36717 (Korean Patent No. 10-0299537) which filed with the Korean Industrial Property Office by the applicant on Aug. 31, 1999. The thin film transistor substrate includes pixel electrodes arranged in a pixel unit, and thin film transistors, each of which is connected to a charging capacitor, a gate line and a data line. The photo sensitive layer produces an electron-hole pair when an X-ray is incident thereto and separates the electron-hole pair when a high voltage of several is applied to the upper electrode. The pixel electrode charges the charging capacitor with holes produced by detection of an X-ray of the photo sensitive layer. The thin film transistor produces a gate signal inputted over the gate line to apply a voltage stored in the charging capacitor to the data line. Pixel signals supplied to the data line are applied, via a data reproducer, to a display device.
FIG. 1 is a sectional view showing a thin film transistor substrate of a conventional X-ray detecting device. The TFT substrate of conventional X-ray detecting device includes a TFT including a gate electrode 13, a gate insulating layer 15, an active layer 17, an ohmic contact layer 19, a source electrode 21 and a drain electrode 23 all on a transparent substrate 11. The conventional TFT substrate also includes a storage capacitor Cst including lower and upper electrodes 29 and 33 and a dielectric layer 31. The TFT substrate further includes a protective layer 35 formed on the TFT and the storage capacitor Cst, and a pixel electrode 37 connected electrically to the upper electrode 33 of the storage capacitor Cst through a contact hole which is formed on the protective layer 35.
The conventional device is arranged at an intersection between a gate line (not shown) and a data line 25. The gate line is connected to the gate electrode 13 and the data line 25 is connected to the source electrode 21. The gate electrode 13 and the insulating layer 15, which cover the gate electrode 13, are formed on the transparent substrate 11. The gate electrode 13 is made from a conductive metal such as aluminum (Al) or copper (Cu) and the gate insulating layer 15 is made from silicon nitride or silicon oxide.
The active layer 17 is formed on the gate insulating layer 15 to overlap with the gate electrode 13, and the ohmic contact layer 19 is formed on the active layer 17 excluding a center portion of the active layer 17. The active layer 17 is made from amorphous silicon or polycrystalline silicon, and the ohmic contact layer 19 is also made from amorphous silicon or polycrystalline silicon. The active layer 17 is not doped, but the ohmic contact layer 19 is doped with either an n-type or p-type impurities at high concentrations.
The source and drain electrodes 21 and 23 are formed on the ohmic contact layer 19 and spaced apart from each other. The source and drain electrodes 21 and 23 are made from molybdenum (Mo), chromium (Cr), titanium (Ti), tantalum (Ta), or from molybdenum alloys such as MoW, MoTa or MoNb, and make an ohmic contact with the ohmic contact layer 19. The source electrode 21 is connected to the data line 25.
The lower electrode 29 of the storage capacitor Cst is formed on the gate-insulating layer 15 and overlaps with a ground line 27. The ground line 27 is formed from the same material as the source and drain electrodes 21 and 23, and is formed by the same process that forms the source and drain electrodes 21 and 23.
The dielectric layer 31 covers the TFT including the lower electrode 29. The upper electrode 33 is formed on the dielectric layer 31 above the lower electrode 29. The dielectric layer 31 is made from silicon nitride or silicon oxide. The lower and upper electrodes 29 and 33 are made from indium tin oxide (ITO), tin oxide (TO) or indium zinc oxide (IZO).
FIGS. 2A to 2E show a process of fabricating a structure having the upper electrode 33 of the storage capacitor Cst on the TFT substrate of the conventional X-ray detecting device shown in FIG. 1. Referring to FIG. 2A, a metal, such as aluminum (Al) or copper (Cu), is deposited on the transparent substrate 11 by the sputtering technique to form a thin metal film. The thin metal film is patterned to form the gate electrode 13, connected to the gate line (not shown), by photolithography including wet etching.
Referring to FIG. 2B, the gate insulating film 15, the active layer 17 and the ohmic contact layer 19 are sequentially formed on the transparent substrate 11 by the chemical vapor deposition (CVD) technique and cover the gate electrode 13. The gate insulating film 15 is formed by depositing an insulation material such as silicon oxide or silicon nitride.
Also, as mentioned above, the active layer 17 is made from amorphous silicon or polycrystalline silicon, and the ohmic contact layer 19 is also made from amorphous silicon or polycrystalline silicon. The active layer 17 is not doped, but the ohmic contact layer 19 is doped with either n or p-type impurities at high concentrations.
The ohmic contact layer 19 and the active layer 17 are patterned by photolithography including anisotropic etching so that a desired portion corresponding to the gate electrode 13 remains.
Referring to FIG. 2C, a metal such as molybdenum (Mo), chromium (Cr), titanium (Ti), tantalum (Ta), or from molybdenum alloys such as MoW, MoTa or MoNb, is deposited on the gate insulating film 15 by the CVD or sputtering technique to cover the ohmic contact layer 19. The metal or the metal alloy so deposited makes an ohmic contact with the ohmic contact layer 19.
Then, the source and drain electrodes 21 and 23 are formed by patterning the metal or the metal alloy by photolithography so that portions corresponding to each side of the active layer 17 remain. At this time, the data line 25 and the ground line 27 are also formed, both of which are perpendicular to the gate line (not shown).
The data line 25 and the ground line 27 are made from the same material as the source and drain electrodes 21 and 23. Further, the ground line 27 is connected to the source electrode 21 (connection not shown).
When the source and drain electrodes 21 and 23 are formed, a portion of the ohmic contact layer 19 between the source and drain electrodes 21 and 23 is patterned to expose the active layer 17. A portion of the active layer 17 above the gate electrode 13 and between the source and drain electrodes 21 and 23 becomes a channel.
Referring to FIG. 2D, a transparent conductive material such as ITO, TO or IZO is deposited on the gate insulating layer 15 covering the data line 25 and the ground line 27. Then the transparent conductive material is selectively removed by photolithography including wet etching to form the lower electrode 29 of the storage capacitor Cst. The lower electrode 29 remains electrically connected to the ground line 27. On the other hand, the lower electrode 29 should not be in contact with the data line 25.
Referring to FIG. 2E, silicon nitride or silicon oxide is deposited on the gate-insulating layer 15 to cover the TFT and the lower electrode 29, thereby forming the dielectric layer 31. The dielectric layer 31 is used as a dielectric film of the storage capacitor Cst.
Subsequently, a transparent conductive material such as indium tin oxide (ITO), tin oxide (TO) or indium zinc oxide (IZO) is deposited on the dielectric layer 31. Then, the transparent conductive material is patterned by photolithography including wet etching to form the upper electrode 33 of the storage capacitor Cst. The upper electrode 33 must be formed directly above the lower electrode 29.
The conventional X-ray detecting device has a problem in that, since the lower electrode of the storage capacitor and the data line are provided on the same plane, a short therebetween may occur during the patterning process due to the presence of residual conductive materials.
Accordingly, it is an object of the present invention to provide an X-ray detecting device and a fabricating method thereof wherein a data line and a lower electrode of a storage capacitor are formed on a different plane to prevent a short therebetween.
In order to achieve these and other objects of the invention, an X-ray detecting device according to one aspect of the present invention comprises a transparent substrate; a gate electrode formed on the transparent substrate; a gate-insulating layer formed on the transparent substrate covering the gate electrode; an active layer formed on a first portion of the gate-insulating layer over the gate electrode; an ohmic contact layer formed on each side of the active layer such that a center portion of said ohmic contact layer is removed; a thin film transistor including source and drain electrodes formed on the ohmic contact layer; a data line formed on a second portion of the gate insulating layer and electrically connected to the source electrode; a ground line provided on a third portion of the gate-insulating layer; a data line insulating layer formed on the gate insulating layer such that the data line is covered and a contact hole is defined in the date line insulating layer exposing the ground line; a lower electrode contacting the ground line via the contact hole on the data line insulating layer; a dielectric layer formed on the data line insulating layer covering the lower electrode; and an upper electrode formed on the dielectric layer above the lower electrode.
The X-ray detecting device according to another aspect includes a thin film transistor (TFT) structure on a substrate including a data line and a ground line; a data line insulating layer formed on said substrate covering said data and ground lines such that said data line insulating layer has a contact hole exposing said data line; a lower electrode of a storage capacitor on said data line insulating layer above said data line such that said lower electrode is electrically connected to said data line via said contact hole; a dielectric layer formed above said lower electrode; and an upper electrode of said storage capacitor formed on said dielectric layer above said lower electrode.
An aspect of a method of fabricating a X-ray detecting device according to another aspect of the present invention comprises the steps of forming a gate electrode on a transparent substrate; sequentially forming a gate insulating layer, an active layer and an ohmic contact layer covering the gate electrode and patterning the ohmic contact layer and the active layer such that the ohmic contact layer and the active layer remain above the gate electrode on a first portion of the gate insulating layer; forming source and drain electrodes on each side of the active layer, the source and drain electrodes making contact with ohmic contact layer and, at the same time, forming a data line and a ground line at second and third portions, respectively, of the gate insulating layer; forming a data line insulating layer covering the data line and the ground line on the gate insulating layer and then patterning the data line insulating layer to define a contact hole for exposing the ground line; forming a lower electrode on the data line insulating layer contacting the ground line via the contact hole above; forming a dielectric layer on the data line insulating layer covering the lower electrode; and forming an upper electrode on the dielectric layer above the lower electrode.
Another aspect of the method to form the X-ray detecting device includes forming a thin film transistor structure on a substrate including a data line and a ground line; forming a data line insulating layer on the substrate covering said data and ground lines such that said data line insulating layer has a contact hole exposing said data line; forming a lower electrode of a storage capacitor on said data line insulating layer above said data line such that said lower electrode is electrically connected to said data line via said contact hole; forming a dielectric layer above said lower electrode; and forming an upper electrode of said storage capacitor on said dielectric layer above said lower electrode.